1. Field of the Invention
This invention relates to composites which are useful as heat storage, materials and which are comprised of a filled ethylene polymer and an organic latent heat storage material.
2. Background
As fossil fuels become scarce and more expensive, the use of alternate energy sources, such as solar energy and waste energy from industrial processes, becomes more attractive. Since the time periods during which such sources of energy are available do not necessarily correspond to the time periods of energy need, energy storage plays an important role in the use of alternate energy sources.
Various types of heat storage are known. For example, heat can be stored in the form of the sensible heat of a fluid such as water or the sensible heat of a solid such as stone or in the form of a combination of sensible heat and latent heat of a transition, particularly the latent heat of fusion, using materials such as inorganic salt hydrates, paraffin or organic polymers. The transition temperature of the latent heat material used must be below the temperature of the material from which heat is to be removed and stored, and equal to or above the temperature which is to be provided on removal of heat from the latent heat material.
Latent heat materials undergoing liquid/solid phase transitions must be suitably contained. One way to accomplish this is to place the latent heat storage material in a container of suitable size, making allowance for temperature expansion of the latent heat material. This approach using a container may give rise to a heat transfer problem when the heat is removed from the latent heat storage material. Solidification of the latent heat storage material occurs first on the walls of the container, and any additional heat removed must be conducted through the solid thus formed, the solid increasing in thickness as more of the liquid solidifies. Conversely, when heat is being stored in a solid latent heat material, heat transfer is inefficient because convection, which is requisite to efficient heat transfer, is hindered by the high viscosity of the liquid first formed from melted solid. Moreover, on cycling, the solid latent heat storage material may contract and pull away from the walls of the container, thereby further decreasing the efficiency of heat conduction.
The art discloses attempts which have been made to solve some of these problems. For example, a granular form of latent heat material may be used, with the heat transfer medium being passed through a bed of such granules either to supply heat or to extract heat from the granules.
U.S. Pat. No. 2,846,421 discloses a method for controlling the temperature of liquid phase reactions, for example, emulsion polymerization, by means of an encapsulated latent heat material, the capsule being formed from a metal or plastic, the latent heat material being commonly available materials, including water, benzene, glycol, mercury, Glauber's salt and Wood's metal. U.S. Pat. No. 4,182,398 discloses a method for removing heat from a fluid by means of crystalline polyethylene silane-grafted-crosslinked polymer pieces crosslinked to retain at least 70% of the heat of fusion of the uncrosslinked crystalline polymer and sufficiently crosslinked for the pieces not to stick together upon being cycled above and below the melting point of the polymer. U.S. Pat. No. 4,221,259 discloses a method for storing heat by means of a fusible substance, that is, a latent heat material, which is absorbed on a finely divided microporous carrier. Paraffin absorbed on active coal or coke in grains or sticks is exemplified. Other latent heat materials which are disclosed are fusible mineral salts, metal hydrides, alloys, metal alloys and polymers. U.S. Pat. No 4,003,426 discloses a heat or thermal energy storage structure comprising a crosslinked polymeric resinous matrix having a plurality of substantially unconnected small closed cavities and a heat sink material encapsulated within the cavities. A similar type of heat storage composition is disclosed in U.K. patent application GB No. 2086032A.
It is an object of this invention to provide a composite material which is suitable for heat storage. Another object is to provide such a material which can be cycled repeatedly between heat sink and heat source conditions without substantial deterioration. Still another object is to provide such a material which can be fabricated readily from commonly available ingredients. Other objects will become apparent hereinafter.